1. Field of the Invention
This invention relates to the thermoforming of articles from plastic material, and specifically in the use and construction of thermally deformable thermoform plastic material sheets used in vacuum molding, and more specifically in the use and construction of thermoform plastic material sheets for use in vacuum molding which are heated by wires internal to the thermoform plastic material sheet.
2. Description of Prior Art
Thermoforming, sometimes referred to as “vacuum molding”, is the process that molds thermoform plastic material sheets into the desired shape through the pressing of “formers” (positive molds) into the previously heated plastic. The process of thermoforming is used in many industries, including the dental industry to form various articles including dental trays, base plates, fluoride trays, prostheses, splints, mouth guards, night guards and custom impression trays.
Thermoforming involves having a positive mold of an article supported between a vacuum-equipped surface, which is sometimes a special table, and a substantially flat, planar thermoform plastic material sheet, made of a plastic material with the desired properties. One such plastic material is EASTMAN Polyester 12822, available from Eastman Chemical of Kingsport, Tenn.
Heat from an external heat source such as a hot air blower, heat lamp or other radiant heat source is directed at the thermoform plastic material sheet. The thermoform plastic material sheet is heated to the point of softening. A vacuum is then applied to and below the table and around the mold thereon, and the now-softened thermoform plastic material sheet is rapidly and suddenly drawn toward the table, thus placing the softened plastic material in contact with the mold surface. The vacuum draws the softened plastic into tight contact with, and conformance to, the contours of the mold surface. Thus, the plastic assumes the shape of the mold. This prior art process is illustrated in FIG. 1A. After the plastic cools, it hardens to a solid, and the resulting article may be removed from the mold, as illustrated in FIG. 1B.
Proper heating of the plastic material is essential to the thermoforming process. Various techniques in the prior art have used a number of devices and methods to accomplish this heating. Typically heat is applied externally by convection, where very hot air is forced against the plastic, or by radiation, where infra-red lamps and/or hot radiating plates are used to heat the plastic, and perhaps sometimes by conduction.
Indeed, it is believed that all prior art devices and methods utilize a heat source that is external to the thermoformed plastic material sheet. However, the use of an external heat source has a number of disadvantages, such as the loss of heat to the environment, inefficiency in transferring heat to the material heated, etc.
An example of external delivery of heat to the thermoform plastic sheet is found in U.S. Pat. No. 6,072,158, which discloses, inter alia, quartz heaters above the thermoform plastic sheet heating it by radiation and hot air blowers below the thermoform plastic sheet heating it by convection.